why is hallow stuff stronger?!?!?!

thnx for the reiteration gilby. i’m not an engineer but i think i got i the first time, but seems that others didn’t so i tried to paraphrase(albeit poorly) for them

I really like Gilby’s explanation. Being as I still have yet to get a chance at going to an engineering school, I’ve always understood things that hollow works because you increase the diameter of the tube in proportion to the drop in strength from losing the center of the tubing. The larger the diameter, the larger the distance that the material is from the center of the axis on which force is transmitted. This means that the same amount of force is spread over a larger area, allowing for a lessened need for material to absorb that force. Correct me if I’m wrong, but isn’t it true that the larger the tubing on a bike frame, the thinner the walls (assuming the same application of the frame).

The new Koxx cranks are solid aluminum, I think.

Not everything is hollow, but that can just be due to limitations of manufacture. Look at some versions of the Steve Howard frames. If there were such thing as a bit that could precisely drill the length of one of his frames, I’m sure he would use it. Also, it is notable how on his frames the edges of the legs are always far more substantial than the centers. This is a 2D version of making something hollow.

Hollow materials are used because of a weight issue, not a strength issue. You can get sufficient (not necessarily equal) strength out of a much lighter design; i.e. hollow instead of solid.

Re: why is hallow stuff stronger?!?!?!

“Gilby” <Gilby@NoEmail.Message.Poster.at.Unicyclist.com> writes:

> This is because … the farther away the material is from the centroid
> axis, the more stress it’ll handle

That’s the right idea, but not precisely so. I’d perfer to say that
increasing the separation of opposing stresses - as happens when you
compare a tube to a rod of the same weight per length - increases the
parts ability to withstand external torques. That’s because torque is
force times distance, as Gliby mentioned:

> similar to having a wrench with a short
> handle verses having a wrench with a long handle

Technically, stress is an internal force per unit area that a part
experiences. Increasing stress for a given load leads to earlier
failure of the part. The trick about hollow shapes is that they can
be stronger and/or lighter without increased stress.

Looking at the “centroid axis” is useful for parts like hubs which
experience far greater stress due to torsion (i.e. twisting along the
axis) than from bending, but in general you can’t analyze strength
using that concept.

Ken

If you do go in to engineering in college you’ll learn all about this in your engineering statics course, probably during your junior year. Or you could get ahold of an engineering statics textbook from the library and try to make sense of the math and the concepts on your own.

And yes, Steve Howard is making a bit of a I beam style structure to give the fork legs some extra strength while keeping the weight down. It’s the 2D equivalent of a hollow tube.